I can assure you that it is not part of there script sometimes it really can be your network card.
however most of the time that is not the case and the tech doesnt have a straight answer for you so they tell you that.
The reason they may think that is if the tech has had you do the following things and you still are getting slow speeds
No packet loss
hard coded the dns
reinstalled/uninstalled the driver for the NIC
Tower is close by
low noise/interferance on your line
Rebooted the modem
cleared the dns
released and renewed the connection
disabled antivirus/firewall software
bypassed any connections in the middle (router, hubs, or clearplugs)
now if the tech had you do all this which they most likely didnt they would say its the network card. however most of them do not know that the modem is a crappy network card and the cord u use that comes with the modem is crappy so it might be those as well.
also the tower could be overloaded as well. Most likely it is not an issue with the NIC but with the clearwire service